The invention is directed to emergency safety systems for aboveground tanks used to store liquid natural gas or similar flammable or otherwise hazardous material. For convenience, the invention will be described with reference to liquid natural gas (LNG).
With the development of synthetic natural gas and liquefied natural gas, storage tanks to hold the liquefied gas at temperatures of approximately -260.degree. F. are required. Various attempts to store the liquefied natural gas below ground to provide long-term storage have been declared failures. Accordingly, most storage at processing plants and receiving storage is in insulated steel tanks above ground. This represents a potential diaster since during an accidental rupture of such a tank, many thousands of, if not millions of, gallons of LNG could spill.
It is normal practice at such tanks to provide dikes surrounding the tanks in the same manner that such dikes are provided for crude oil and other petroleum product tanks, though design criteria for such dikes are rather general and the dikes cannot contain the vaporized LNG. During a rupture or leak in the storage tank, it has been expected that the diked area will contain the flammable liquid to prevent the liquid from spreading to adjacent facilities on or off the storage area where it might be ignited. Adjoining properties are protected only by an exclusion distance from the dike.
However, unlike other petroleum products, LNG boils at a temperature of -260.degree. F. and thus LNG in the diked area will evaporate quickly while rapidly cooling the structures it is in contact with. The evaporated highly flammable gas is initially heavier than air and will flow as a vapor out of the area surrounding the tank while hugging the ground surface. Dikes have been proposed, but although the dike will prevent the escape of the liquid to the surrounding facilities, it can not prevent the spread of the gases which are also very flammable and, therefore, dangerous.